When typing a sign and leaving the rest of the line blank, The word "Null" will appear.

To Catch this Bug:
1. Create a new world in 1.7.
2. place a sign and type what ever you want to type.
3. Save and Quit
4. Launch Minecraft with Minecraft 14w28b.
5. Open the new world you have just created
6. You catch the bug from there